In microwave communications, the strength of a microwave signal can decrease as a result of communication channel impairments due to natural causes such as precipitation, humidity, or terrain and man-made causes such as structures which scatter or block the microwave signal. In some situations the decrease in signal strength prevents reliable communication. Diversity provides multiple opportunities to access the microwave signal and improve the probability of reliable communication. The multiple opportunities to access the microwave signal may be implemented by exploiting redundancies in the time, frequency and/or field domains of the signal, where field domains consist of the spatial, polarization, and radiation pattern attributes of the signal.
A single dual-mode patch antenna, which is a microstrip antenna excited to generate two orthogonal polarizations, has been used for diversity in Motorola's 2.45 GHz radio local area network, RLAN. The use of a single-mode patch or similar antennas known in the art such as an inverted-F antenna together with a whip antenna is common practice for obtaining field diversity on portable radio handsets, especially in the Japanese cellular arena.
Some emerging 1.9 GHz personal communication systems, PCSs, such as the Personal Access Communications System, PACS, air interface require that the subscriber unit provide field diversity for both transmit and receive. Typical full-duplex radios with this requirement would employ an antenna switch to select from one of the two antennas providing the field diversity and a diplexer that operates to reduce the coupled energy from the transmitter to the receiver. In a two frequency full-duplex system, diplexing allows a transmitter signal and a receiver signal to be coupled in a manner that does not degrade either signal. With knowledge of the filter impedance characteristics, controlled length transmission lines are used to provide the proper impedance for both transmitter and receiver filters. This impedance isolation is necessary for efficient operation. The filters provide signal isolation by reducing the amount of receiver signal lost to the transmitter and the amount of transmitter signal lost to the receiver. This diplexing operation imposes constraints on the circuit board layout and adds complexity to the transmit and receive filter designs, generally leading to increased insertion loss and the requirement for controlled-phase-length transmission lines between the filters. Time-duplexed systems could replace the diplexer with a second switch to select transmit or receive, but this adds an additional insertion loss to both the transmit and receive paths.
Accordingly, there is a need for a method, dual rectangular patch antenna system, and radio for providing isolation and diversity while eliminating the need for a diplexer or a second transmit/receive switch.